1. Field of the Invention
This invention relates to techniques for curving glass sheets by settlement or sinking onto a curving frame. It concerns, more specifically, an installation making possible very accurate control of the heating conditions of glass sheets for the purpose of complete mastery of the principal and secondary curvatures which it is desired to impart to the glass sheet. The invention is applicable, notably, to the production of laminated glazing intended for automobile vehicles.
2. Description of the Related Art
For curving pairs of glass sheets for the purpose of constructing laminated panes, it is usual to arrange the two glass sheets on a curving frame, also known as a skeleton, and to cause the frame to advance continuously or intermittently through a series of heating stations, in which the frame dwells for a greater or lesser time. In electrical furnaces, each of these heating stations is lined with resistors composed, for instance, of tubes of refractory ceramic around which electrical wires are wound. The tubes fitted to the hearth and vault of the furnace are mounted parallel to the furnace axis, whereas the panes are introduced perpendicularly to this axis; in this way it is very easy to provide additional heat to the lateral edges of the pane to which it is generally desired to impart a rather more pronounced curvature than that given to the central part of the pane. Moreover, an orientation of this type biases the optical quality in the sense that any optical defects generated by heating which is not rigorously uniform, but takes place along a series of bands corresponding to the resistor tubes, extend substantially vertically after the pane has been mounted in the vehicle and do not interfere at all with the view of the driver.
Methods of this class are well suited to the production of panes of the type known as cylindrical, in which there is therefore only a single radius of curvature in the horizontal plane. The development of automobile body shapes, however, is leading to a need for panes having a double curvature, that is to say with a secondary curvature in a direction substantially perpendicular to the direction of the principal curvature, which for example allows continuity to be achieved between the roof of the vehicle and its windscreen. This secondary curvature becomes all the more difficult to achieve as the bend becomes more marked--and therefore the smaller the radius of curvature becomes--and/or the nearer it is situated to the edge of the pane. In such oases, it is necessary to provide additional heating for the pane in these zones of high curvature without increasing the curvature of the central part of the pane, which adopts a spherical deformation. Although it may be possible, if really necessary, to produce panes of this type with the methods of the state of the art, notably by subdividing the electrical resistors of the edge zones of the installation, it is virtually impossible to obtain panes of the type known as "S", that is to say possessing, in addition to their principal curvature, two secondary radii of curvature in opposite directions. Such "S"-shaped panes are suitable for the construction of an automobile in which the windscreen is tangential to both the hood (bonnet) and the roof.